The third movement of Mozart's Sonata in C, K.330 (K.V.300h) is a graceful Allegretto in sonata form. Interestingly, the last four bars were apparently added later.
This is one of the composer's most popular sonatas, apparently first improvised during a concert in Augsburg on October 22, 1777. Mozart wrote, "I then played... all of a sudden a magnificent sonata in C major, out of my head, and a Rondo to finish up with." He did write it down later and it was published by Artaria in 1784 as Opus 6.